Fig Futaba FASST spectrum as seen on an
HP141T with the storage display turned on.

As an radioamateur with interest in microwaves I had to
make some investigation of the Futaba FASST system – I did
choose FASST just because i already owned a Futaba T9CP with a 35 MHz
module, the easiest way of getting the better realiability of 2,4 GHz
system was choosing the Futaba plug in system with a TM7 transmitter
module.

It is a FHSS and DSSS system but I have not been able
to find any futher data on how the radiolink works.

Some
measurements has been done by using an Analog Devices demodulating
logarithmic amplifier (power detector) ADL5513 and PC soundcard-scope
(by C Zeitnitz)

The transmitter is transmitting with 1.75 ms long
pulses and it is 8 ms between each TX-pulse. (the frequency is
different for each TX-pulse)

Fig. TX pulse 1.75ms duration, 8ms between
each TX-pulse.

The frequency hopping ?

It seemsthat
Futaba is using 36 frequencies (in general mode, i have not tested
the mode for France) from 2404.5 MHz to 2476.5 MHz.

How to detect the frequency hopping ? – as I dont have
access to the latest Rohde & Schwarz realtime spectrum analyzer
FSVR – which anyway do not cover the entire 2.4 GHz ISM band
with its 40 MHz realtime BW, this has to be done in an other way.

I retuned an old interdigital filter (built for some ATV (Amateur
Telvision) tests that I have done some years ago) to acchieve an
amplitude slope over the band of interest so that the frequency
hopping will be amplitude modulated and thus can be detected with the
logaritmic amplifier.

Fig.
Filter performance with 3 dB attenuator at input and output, the slope is approx
40 dB over the band of interest.

On the PC-scope it is now possible to see the frequency hopping
behaviour. The sequence is changing between every time the
transmitter is turned on., see the figures below. It is somewhat
difficult to see the length of the sequence but it looks like 288 ms
(which makes sense as 8ms x 36 frequencies = 288ms). How many
different hopping sequencies ? I have no idea !

Fig TM7
frequency hopping - different hopping pattern for every restart of the
module.